1. Field of the Invention
The present invention relates to a ceramic circuit board and a method of manufacturing the same in which a conductive layer as a circuit is integrally formed to a ceramic substrate, more particularly to a ceramic circuit board and a method of manufacturing the same which can realize a ceramic circuit board having an excellent heat radiating property and a high strength which is capable of reducing crack formation during assemble and operation of the circuit board, and is capable of reducing short-circuit accident to be occurred in the conductive layer.
2. Description of the Related Art
Conventionally, as a part constituting electronic equipments and semiconductor devices, there have been widely used various ceramic circuit boards (thick-film circuit board) 1 as shown in FIGS. 1 and 2. The ceramic circuit board 1 is manufactured by integrally forming a conductive layer (metallized wiring layer) 3 as a circuit to a surface and/or an inner portion of a ceramic substrates 2 such as alumina (Al.sub.2 O.sub.3) substrate, aluminum nitride (AlN) or the like.
As the ceramic substrate constituting the conventional ceramic circuit board, the alumina (Al.sub.2 O.sub.3) substrate having a thermal conductivity of about 10-20 W/m.multidot.K has been widely used. Further, in case of a use where a higher heat radiating property is demanded, there has been used a structure in which heat radiating plates having various shapes and heat-sinks are combined with the circuit boards. Furthermore, there is a case where aluminum nitride (AlN) substrate having a thermal conductivity of 50-150 W/m.multidot.K is used thereby to secure a higher heat radiating property of the ceramic circuit board.
In recent years, as the progress of miniaturization and high-densification of the electronic equipments, the semiconductor parts are required to be applied with a large current for realizing a high power operation, so that there is a tendency of increasing an amount of heat to be generated from the semiconductor parts. Therefore, how to efficiently release the heat generated from the semiconductor parts has become a serious and important problem.
An aluminum nitride has excellent characteristics such as a high thermal conductivity, a good electrical insulating property and a thermal expansion coefficient which is almost the same as that of silicon (Si), so that aluminum nitride is most suitable for the substrate material for the semiconductor parts.
By the way, when the aluminum nitride (AlN) is used as a semiconductor substrate, it is necessary to form a metallized layer composed of electrically conductive metal onto a surface of the AlN substrate for the purpose of forming a circuit and a mounting portion for mounting the electronic parts or the like. As one method of metallizing, a simultaneous sintering method (co-fire method) has been utilized. In general, the metallized layer is formed on a surface of the substrate in accordance with the following procedure.
That is, a binder agent composed of organic compound is added to aluminum nitride (AlN) to prepare a slurry, then the slurry is molded by a doctor-blade method thereby to obtain an AlN green sheet having a sheet-shape. Thereafter, a refractory metal paste containing W or Mo or the like is printed onto the surface of the AlN green sheet to form a molded body. Thus obtained molded body is heated and degreased, thereafter sintered in a non-oxidative atmosphere thereby to obtain an AlN sintered body.
In addition, in recent years, a high-power operation of semiconductor device using a ceramic circuit board, a large-capacity and a high-integration of a semiconductor element have been rapidly advanced, so that there is also a tendency of rapidly increasing thermal stress and heat load to be applied to the ceramic circuit board. Therefore, the ceramic circuit board is required to have a sufficient strength and heat radiating property against the above thermal stress and heat cycles.
In order to cope with the above technical requirements, there has been also developed an aluminum nitride (AlN) substrate having a high thermal conductivity of about 180 W/m.multidot.K. This aluminum nitride (AlN) substrate is manufactured by a method comprising the steps of: adding sintering agent (sintering aid) such as yttria (Y.sub.2 O.sub.3) or the like to high-purity AlN material powder to prepare a material mixture; molding the material mixture to form a molded body; and sintering and firing the molded body at a high temperature exceeding 1800.degree. C. for a long time of about 48-72 hours so that the molded body is sintered and densified, simultaneously a liquid-phase component to become a heat resistance is discharged toward the surface of the substrate to realize a high purity of the substrate.
In the conventional ceramic circuit board using the above ceramic substrate, a high thermal conductivity could be obtained by improving a kind of the ceramic substrate or the sintering or firing methods. However, since the heat treatments (sintering and firing) for the ceramic material powder were conducted for a long time, the resultant ceramic crystal grains became coarse due to grain-growth. As a result, sufficient heat-cycle durability and bending strength could not be obtained, whereby there was posed a problem that reliability and production yield of the semiconductor device using the ceramic circuit board was disadvantageously lowered.
That is, in accordance with the progress of the high-integration and the high-power operation of the semiconductor element mounted on the ceramic circuit board, a heat-cycle load was remarkably increased. As a result, there was posed a problem that cracks would occurred at the ceramic substrate whereby a function of the circuit board was completely lost.
In addition, since the bending strength of the circuit board was small and a deflection amount thereof was also small, when the circuit board was fixed to a mounting board by means of screws at the time of assembling a semiconductor device, there was a case of that the ceramic substrate was broken by a slight fixing force of the screw, thus raising a problem of lowering a production yield of the semiconductor device using the ceramic circuit board. Furthermore, there were many cases where cracks occurred by a thermal stress to be caused during the operation of the circuit board, thus being a disadvantage of lowering the reliability of the semiconductor device.
In addition, the conventional method adopted a procedure in which the additives such as sintering agent or the like were discharged as liquid-phase component (glass component) toward a surface of the ceramic substrate or the conductive layer by firing treatment for a long time of period, thereby to improve the purity and to increase the thermal conductivity. Therefore, the liquid-phase component was non-uniformly existed on the ceramic substrate or the surface of the conductive layer, whereby there was posed a problem of exerting a bad influence on the surface treatment for the ceramic substrate. Namely, irregularities or micro-defects would occur at the surface of the ceramic substrate or the conductive layer due to the liquid-phase component. Therefore, an excessive plating component adhered to the defectives or the like remained at portions between adjacent conductive layers when the surface of the conductive layer was subjected to a plating treatment, so that there were posed problems such that short-circuits occurred, the plating layer was non-uniformly formed and a solder-wetted area was non-uniformly formed at a time of the solder-reflow operation, thus making the effect of the surface treatment insufficient.
In order to obtain an AlN substrate having a thermal conductivity of 180 W/m.multidot.K or more, it is necessary to sinter the AlN molded body at a sintering temperature exceeding 1800.degree. C. However, when the AlN molded body is sintered at the temperature exceeding 1800.degree. C., there has been posed a problem such that a glass component as liquid phase component contained in the AlN substrate bleeds out toward the surface portion of the AlN substrate, or the glass component would penetrate into the AlN substrate or the like.
When the glass component contained in the AlN sintered body bleeds out toward the surface portion of the AlN substrate, the bleedings become a cause of stains and discolorations to be formed at the surface of the AlN substrate. Further, when the glass component penetrates into the metallized layer or bleeds out to the surface of the metallized layer as the conductive layer, there are caused problems such that an appearance of the metallized portion becomes bad to constitute a defective and the soldering property is lowered, so that it becomes impossible to form a circuit on the metallized layer or to mount the electronic parts. That is, there has not been obtained an AlN circuit board which is manufactured by the simultaneous sintering method so as to have a thermal conductivity of 180 W/m.multidot.K or more and a metallized layer formed on a ceramic substrate.
For this reason, there has been conventionally adopted a post-fire method in which the molded body is previously sintered at a high temperature exceeding 1800.degree. C. to prepare an AlN substrate having a thermal conductivity of 180 W/m.multidot.K or more, then the glass component bled out toward the surface of the AlN substrate is removed by grinding work or the like, thereafter, a refractory metal paste is coated onto the surface of the AlN substrate and the coated paste is baked.
In case of this post-fire method, however, since man-hour (labor cost) is greatly increased in comparison with that of the simultaneous sintering method (co-fire method) and the glass component is required to be removed by the grinding work or the like, there is posed a problem such that fine wirings cannot be formed, etc. Further, the AlN substrate is required to be formed so as to have a larger thickness containing a grinding margin, thus being troublesome and raising a problem of lowering the production efficiency.
Furthermore, other than the metallizing method using the post-fire method, there has been adopted a method in which a thin-film is formed on the surface of the substrate. Examples of the method may include a thin-film forming method by vapor deposition, a sputtering method which is one kind of vacuum evaporation methods, and CVD (chemical vapor deposition) method or the like. In these methods, however, there are posed problems such that the number of manufacturing steps is further increased in comparison with that of post-fire method, and the grinding work for the substrate surface is also required as the same as in the post-fire method.